In the RF transmitter portion of a transceiver data is typically scrambled, encoded, and interleaved before being modulated and then up-converted and amplified for transmission. The receiver portion of the transceiver performs the reverse processes including down-converting the RF received signal to baseband.
Conventional single step (also referred to as direct carrier modulation) up-converters have been unable to achieve satisfactory suppression of the undesired image sideband which, as a result of the up-conversion process performed by a mixer, is generated in addition to the desired sideband. Equally, such conventional single step up-converters have been unable to achieve satisfactory suppression of any local-oscillator feed through.
Suppression of the image sideband can be achieved with the use of a high Q, off-chip RF filter; however to achieve low loss and stable frequency, this presents a high cost solution.
The use of two-step up-converters (a first one used to up-convert to an IF carrier and a second one used to up-convert to the RF carrier) is one known means of suppressing the image sideband while avoiding the foregoing requirement of single step up-converters for a high Q, off-chip RF filter. Upon a first up-conversion to an IF carrier the image sideband is removed by means of a low Q filter. Then the desired IF sideband is up-converted to an RF carrier and another low Q filter is used to suppress the resultant image sideband and any local oscillator feed through. Although the cut-off bandwidth of each of the filters used in such IF and RF circuits is higher than that required for a single step up-converter, which makes these filters more easily realizable, they are nevertheless high cost, off-chip components the use of which is, preferably, to be avoided.
Another known means of suppressing the image sideband is provided by the conventional image rejection up-converter which splits the baseband signal into two quadrature components, each of which is individually up-converted with a quadrature local oscillator and then the resulting up-converted signals are combined. The undesired image sideband is suppressed in the resultant combined signal without need for an RF filter. However, this conventional image rejection up-converter does not suppress any local oscillator feed through and requires a high degree of phase and amplitude balance in the two up-converters.
The foregoing disadvantages associated with conventional single step and two-step up-converters also apply to their corresponding single step and two-step down-converters.
Accordingly, there is a need for an improved up-converter to provide effective and efficient suppression of any local oscillator feed through. In addition, there is a need for an improved up-converter and down-converter to provide effective and efficient rejection of the image sideband.